Previously, cDNAs were constructed to encode "minigenome" RNA, an internally-truncated version of the genomic RNA of respiratory syncytial virus (RSV). In the minigenome, the authentic genomic termini are retained but the viral genes are deleted and replaced with one or more reporter genes, such as that for bacterial chloramphenicol acetyl transferase (CAT) or luciferase (LUC), each under the control of a set of the RSV sequence motifs which are thought to control transcription. When transfected into RSV-infected cells, the prototype RSV-CAT minigenome was replicated, transcribed and packaged into particles which could be passaged to fresh cells. Here, this system was used to identify and characterize the cis-acting replication and transcription signals in the RSV genome and to study the mechanism of gene expression.